FIG. 215.—THE CATHODE RAY.
Every student of physics remembers the old-time lecture room experiments in which the Geissler tubes, with their beautiful play of colored lights, illustrated the action of the electrical discharge from the glass plate machine or the Ruhmkorff coil, on rarified gaseous media. Electrical experiments in high vacua by Sir William Crookes, and by Hittorf and Lenard, have greatly added to the present knowledge in this field, and paved the way to the discovery of Prof. Roentgen. It was known that a vacuum tube, variously called after the names of these scientists, as a Crookes, Hittorf, or Lenard tube, having platinum electrodes sealed in its ends, would, under the static discharge of electricity through it, give peculiar manifestations of light. One of the conducting terminals of such tubes was called, in electrical parlance, the “anode,” from the Greek ανα (up) ὁδος (way), meaning the way up or into the tube, and referring to the entering path of an electric current, or its positive pole; while the other was called the “cathode,” from κατα (down), ὁδος (way), meaning the way down or out, and referring to the outgoing path of an electric current, or its negative pole. When such glass tube, partially exhausted of air, received through its anode and cathode terminals a discharge of static electricity, a peculiar manifestation of light is seen between the anode and cathode terminals. At the anode it appears as a peach blossom glow, and at the cathode it appears as a bluish green light. If the exhaustion of the air in the tube is carried very high, approaching a perfect vacuum, or to about one millionth of the atmospheric pressure, the glow light at the anode disappears, and that at the cathode increases until it fills the entire tube with its characteristic light. This is called the “cathode ray,” or “cathodic ray,” an illustration of which is given in [Fig. 215], where the cathode ray is seen in a Crookes tube emanating from the negative pole N or cathode a, and casting a shadow of the Maltese cross b into the end of the tube, as seen at d. Many of the characteristics of the cathode ray had been observed prior to Prof. Roentgen’s discovery, which, briefly stated, grew out of the following observation: He noticed that when a vacuum tube illumined by the cathode ray was completely masked or covered up by an external shield of black paper, so that no illumination of the tube was visible to the eye, there still passed through it certain subtle rays of light, invisible to the eye, but which would instantly illuminate a sheet of paper coated on one side with barium platino-cyanide, even at a distance of two yards or more, and that these invisible light rays were capable of passing through many substances opaque to ordinary light. He also discovered that these rays could be made to take a shadow photograph on a sensitive plate without even exposing the plate in the usual way, the X-Rays passing freely through the opaque ebonite or pasteboard screen of the plate holder. It did not take the scientific world long to realize the immense importance of this discovery, and to-day X-Ray apparatus constitutes the greatest addition to the surgeon’s resources that has ever been made in the form of mechanical appliances, since by its aid any foreign body in the human frame of greater density than the flesh may be at once definitely located and extracted, or any fracture of the bone disclosed, as the case may be. In the illustration, [Fig. 216], is shown an X-Ray photograph of the hand of a gentleman whose thumb bone has been destroyed by disease.
FIG. 216.—X-RAY PHOTO OF HAND, SHOWING DISEASED THUMB BONE.
Soon after the announcement of Prof. Roentgen’s discovery, apparatus was devised for seeing with the naked eye the image formed by the shadow of the X-Rays. Prof. Salvioni constructed such a device and described it before the Rome Medical Society as early as February 8, 1896. He called it the “cryptoscope.” It was quite a simple affair, and consisted of an observation tube with a lens, having in front of it a screen of fluorescing material, such as platino-cyanide of barium. When the object to be examined, the hand, for instance, was held in front of the fluorescing screen, and the X-Rays from the vacuum tube fell upon the hand, located between the vacuum tube and the fluorescing screen, a shadow of the bones was cast on the fluorescing screen by virtue of the greater density of the bones, which shadow was clearly discernible to the eye at the end of the observation tube. By this device one was able to see his own bones through the flesh. A device, invented by Edison and called the “fluoroscope,” was constructed on substantially the same principle. This used a tapered observation tube like the old-fashioned stereoscope box, which had at its outer wide end the fluorescing screen, and its small end fashioned to fit the forehead and strapped thereto so as to enclose both eyes. This device is shown in [Fig. 217], in which an X-Ray vacuum tube is housed in a wooden box, on which the hand of the patient, or other part to be viewed, is laid, the X-Rays passing readily through the top of the box and casting a shadow of the bones of the hand, or foreign body, on the fluorescing screen of the observation tube. Edison’s experiments also led him in constructing his fluorescing screen, after testing a great number of substances, to select the chemical known as calcium tungstate, instead of the barium platino-cyanide, since the calcium tungstate appeared to give better results in fluorescing. Many other chemicals can be used, however, for making the fluorescing screen, such as the sulphides of calcium, barium and strontium. A recently discovered and powerful fluorescing substance is the double fluoride of ammonium and uranium, discovered by Dr. Mecklebeke. Such fluorescing materials are spread in a thin layer on the side of the screen next to the observer in the viewing apparatus.
